1. Field of the Invention
The present invention relates to a filter adjusting circuit which adjusts the characteristics of a filter and is provided for an active filter.
2. Background Art
In recent years, active filters working on wider frequency bands have been demanded. In active filters for wide frequency bands, however, it is difficult to achieve high quality factors (Q factors) as designed without variations.
Q factors fluctuate due to variations in passive elements such as R, C and L and also nonideal effects of amplifiers in active filters. Although an ideal amplifier has an infinite gain and band, gains and bands are finite in actual amplifiers.
For example, a filter adjusting circuit of the prior art includes a filter where a filter input and a reference clock are switched and inputted and a phase comparator circuit which compares the output of the filter and the phase of the reference clock.
The filter adjusting circuit configured thus adjusts the cutoff frequency of the filter (for example, see Japanese Patent Laid-Open No. 2003-124784).
However, the prior art only discloses a method of adjusting the cutoff frequency of a gm-C filter and does not disclose any specific circuit configurations for adjusting the Q factor of an active filter.
In other words, the Q factor cannot be automatically adjusted in the prior art, for example, when the Q factor varies with manufacturing conditions and the like or when the Q factor fluctuates due to a parasitic effect or a nonideal effect of an amplifier.
As described above, in the filter adjusting circuit of the prior art, it is difficult to design the Q factor of the filter to a desired value.